<template>
  <transition :name="transitionName">
    <div v-show="visible" :style="customStyle" class="back-to-ceiling" @click="backToTop">
      <svg width="16" height="16" viewBox="0 0 17 17" xmlns="http://www.w3.org/2000/svg" class="Icon Icon--backToTopArrow" aria-hidden="true" style="height: 16px; width: 16px;">
        <title>回到顶部</title>
        <g>
          <path d="M12.036 15.59c0 .55-.453.995-.997.995H5.032c-.55 0-.997-.445-.997-.996V8.584H1.03c-1.1 0-1.36-.633-.578-1.416L7.33.29c.39-.39 1.026-.385 1.412 0l6.878 6.88c.782.78.523 1.415-.58 1.415h-3.004v7.004z" fill-rule="evenodd"/>
        </g>
      </svg>
    </div>
  </transition>
</template>

<script lang="ts">
import { Component, Vue, Prop } from 'vue-property-decorator';

@Component
export default class BackToTop extends Vue {
  @Prop({ default: 400 })
  private visibilityHeight!: number;
  @Prop({ default: 0 })
  private backPosition!: number;
  @Prop({ default: () => ({
    right: '50px',
    bottom: '50px',
    width: '40px',
    height: '40px',
    'border-radius': '4px',
    'line-height': '45px',
    background: '#e7eaf1',
  }) })
  private customStyle!: {
        [key: string]: any,
        right: string,
        bottom: string,
        width: string,
        height: string,
        background: string
        };
  @Prop({ default: 'fade' })
  private transitionName!: string;

  private visible: boolean = false;
  private interval: any = null;
  private isMoving: boolean = false;

  public mounted() {
    window.addEventListener('scroll', this.handleScroll);
  }

  public beforeDestroy() {
    window.removeEventListener('scroll', this.handleScroll);
    if (this.interval) {
      clearInterval(this.interval);
    }
  }

  private handleScroll() {
    this.visible = window.pageYOffset > this.visibilityHeight;
  }

  private backToTop() {
    if (this.isMoving) return;
    const start = window.pageYOffset;
    let i = 0;
    this.isMoving = true;
    this.interval = setInterval(() => {
      const next = Math.floor(this.easeInOutQuad(10 * i, start, -start, 500));
      if (next <= this.backPosition) {
        window.scrollTo(0, this.backPosition);
        clearInterval(this.interval);
        this.isMoving = false;
      } else {
        window.scrollTo(0, next);
      }
      i++;
    }, 16.7);
  }

  easeInOutQuad(t: number, b: number, c: number, d: number) {
    if ((t /= d / 2) < 1) return c / 2 * t * t + b;
    return -c / 2 * (--t * (t - 2) - 1) + b;
  }
}
</script>

<style scoped>
  .back-to-ceiling {
    position: fixed;
    display: inline-block;
    text-align: center;
    cursor: pointer;
  }

  .back-to-ceiling:hover {
    background: #d5dbe7;
  }

  .fade-enter-active,
  .fade-leave-active {
    transition: opacity .5s;
  }

  .fade-enter,
  .fade-leave-to {
    opacity: 0
  }

  .back-to-ceiling .Icon {
    fill: #9aaabf;
    background: none;
  }
</style>
